1. Field of the Invention
The present invention relates to a vibration suppression apparatus and vibration suppression method.
2. Description of the Related Art
Recently, as the circuit patterns of semiconductor integrated circuits are becoming finer and increasing the degree of integration, higher accuracy is requested of lithography apparatuses such as an exposure apparatus using the lithography technique. To increase the accuracy of the lithography apparatus, vibrations propagating from the outside such as a base to the lithography apparatus need to be suppressed because they decrease the overlay accuracy and resolution (transfer) performance.
Since a vibration suppression apparatus used for the lithography apparatus generally uses an air spring and has a natural frequency of about 2 to 3 Hz, it cannot suppress vibrations of low frequencies (1 Hz or less) such as vibrations (walking-induced vibrations) caused by movement of a person and vibrations of a building. The vibration suppression apparatus therefore needs to suppress not only high-frequency vibrations but also low-frequency ones. To suppress low-frequency vibrations, Japanese Patent Laid-Open No. 2007-522393 proposes a vibration suppression apparatus including two objects supported in series on a base by elastic supports, a sensor which measures the relative distance between the two objects, and an actuator which drives the base-side object.
The vibration suppression apparatus described in Japanese Patent Laid-Open No. 2007-522393 sets, as a reference distance, a relative distance between the two objects that has been measured by the sensor for the first time, and controls the actuator to reduce the deviation between a relative distance between the two objects, and the reference distance. However, when the sensor measures the relative distance between the two objects for the first time to set a reference distance, an offset may be generated between the steady value of the elastic support (the length of the elastic support when the elastic support does not vibrate) between the two objects, and the set reference distance. While the offset is generated, the actuator drives the base-side object to reduce the offset. Thus, although the relative distance between the two objects is maintained at the reference distance, the distance between the base-side object and the base changes at a uniform acceleration. As a result, the distance between the base-side object and the base may exceed the movable range of the elastic support interposed between the base-side object and the base, the elastic support may be broken, and the vibration suppression apparatus may not be able to suppress vibrations anymore.